The present invention relates generally to simultaneous multiple dimension signal processing, and more particularly to simultaneous optical multiple-dimension signal processing.
Multiple-dimension instantaneous optical signal processors perform instantaneous measurements of signal parameters associated with applied radio frequency signals. In the prior art, multi-dimensional signal parameter measurements are made independently on a narrow band basis. Such instantaneous parameters as frequency bearing, elevation, polarization, etc., are measured by parallel receiving circuits. Accordingly, the number of receivers and processors required to perform multiple parameter analysis is significant.
One prior art technique has been developed for performing one-dimensional optical signal processing. This technique utilizes an elementary form of an acousto-optic processor based upon Bragg optical deflection. An elementary acousto-optic processor is shown in FIG. 1. The basic feature of this processor involves the interaction of acoustic energy propagating in an elastic, optically transparent medium 10 illuminated by coherent light 12 from a laser source 14. The optically transparent medium 10 is an acoustic wave device which acts as the deflection element (Bragg cell) and includes an attached transducer coupler 16 at one end thereof. In operation, RF signals on line 18 from the environment are coupled via the transducer 16 into the acoustic wave device 10. The transducer 16 acts to propagate the RF signal in the acoustic wave device in the form of an acoustic wave which alternately compresses and then expands the material in the acoustic wave device, as it propagates in one direction. This density modulation occurs at the RF drive frequency on line 18 and causes local changes in the index of refraction, thereby inducing an optical diffraction grating in the material. When this defraction grating is illuminated at a precise angle, the Bragg angle from the normal to the direction of acoustic propagation, then optical reinforcement occurs and some of the incident light is deflected onto a given focal plane 20 at an angle proportional to the acoustic frequency. That is the incident and exit angle .alpha. of the light relative to a perpendicular to the acoustic column 22 is ##EQU1##
where .lambda. is the light wave length and .LAMBDA. is the acoustic wavelength. Since the Bragg angle .alpha. is a function of the acoustic wavelength .LAMBDA., a change in frequency at the RF input line 18 will change the physical dimension of the acoustic wavelength .LAMBDA. and result in a new deflection angle .alpha. and a new location on the focal plane 20. Note that since the light source 14 has a narrow beam width, the range of angles and corresponding radio frequencies over which the conditions for Bragg angle deflection are met (the bandwidth) is determined by the acoustic angular beamwidth, i.e., the width of the grating pattern 22 generated by the acoustic transducer. A wide bandwidth acousto-optic processor will act to separate overlapping frequency signals into a number of different deflected optical beams each representing the spectral characteristics of the applied signal or signals. In essence, since the Bragg angle is a function of the acoustic-wavelength, signals applied at different frequencies result in a spatially diverse optical deflection on the focal plane 20.
The acousto-optic processor of FIG. 1 is a one-dimensional processor for processing frequency. The acousto-optical processor may be extended to a second dimension by the use of a planar acousto-optic acoustic wave device and by using a focal plane sensor with a two-dimensional detector array. Such a two-dimensional processor is shown in FIG. 2. One application of this device, frequently referred to as a phase interferometer acousto-optic processor, is described in more detail in the article by A. E. Spezio, entitled "Acoustooptics for Electronic Warfare Applications", "The Proceedings at the 12th Asilomar Conference on Circuits, Systems, and Computers", Nov. 6-8, 1978. The device comprises a planar acoustic wave device 40 with a set of multiple transducers 45 linearly positioned on one edge of the acoustic wave device 40 (See FIG. 3. Note that only the input lines 43 thereto are shown in FIG. 2.). In this example, the RF signal inputs from an antenna array such as, for example, an azimuth antenna array, comprising the antenna elements 30, 32, 34, and 36 are applied through a front end circuit 38 typically comprising standard circuitry for demultiplexing and converting the incoming RF signal to the IF frequency of the acoustic wave device 40. The signals from the front end circuit 38 separately drive via lines 43 the aforementioned transducers on the edge 41 of the acoustic wave device 40. The signals applied to the transducers induce four planar optical diffraction gratings 42 propagating upward in the figure. The entire Bragg cell acoustic wave device 40 is then coherently illuminated with laser source 11 so that laser light propagates into and through the broad planar face of the Bragg acoustic wave device 40 at the Bragg angle. The deflected light output is focused via a lens 44 onto a two-dimensional focal plane 46. This two-dimensional focal plane may be a two-dimensional detector array.
If all of the RF transducers on the edge 41 of the acoustic wave device 40 are excited with in-phase signals, representing a target emitter directly on boresight for the antenna array, then the compressions and extensions caused by the acoustic wave may be represented by the illustration of FIG. 3 (a). With this type of boresight in-phase excitation, deflection of the light occurs as a point only along the vertical axis in the focal plane. This deflection is represented by the vertical boresight line 50 in the focal plane shown below FIG. 3 (a).
However, if the target emitter is located at some off-boresight angle, then there is a linear phase variation across the RF signals applied to the transducers 45. These phase differential RF signals induce segmented planar diffraction gratings across the face of the acoustic wave device 40 such as that shown in FIG. 3 (b). A phase gradient relative to the input transducer edge then results. When these segmented planar diffraction gratings are coherently illuminated by the laser source 14, optical deflection occurs both along the vertical frequency axis 50 and along the horizontal phase axis 52. This type of deflection is represented in the focal plane 20 shown below FIG. 3 (b). In essence, the vertical deflection in the focal plane results from the vertical separation of the diffraction grating lines in the acoustic wave device and indicates the input frequency. The horizontal deflection results from equivalent horizontal separation of the diffraction line gratings and indicates a direction of arrival.
FIG. 4 represents a two-dimensional CRT display of a visual focal plane analysis obtained from a 2D digital processor. In this figure, the direction of arrival in relation to the antenna boresight is indicated by the position of the highest intensity lobe illumination, while the frequency is indicated by the position of the deflection along the frequency axis 62. In this example, three signals are simultaneously applied to the processor. The upper signal is the image of a 1 .mu.sec pulsewidth, 5 kHz repetition rate 1st pulse train on antenna boresight. The boresight position is shown by the location of the highest intensity lobe 60 on the azimuth axis 66 for a first frequency excitation. The lower two signals are continuous wave signals separated by 75 MHz at an angle of 22.degree. off of boresight. The main lobes 64 and 65 for these second and third frequencies are positioned a predetermined amount from the boresight axis 62 to indicate this 22.degree. angle off of boresight.
Note that the above described acousto-optic processor provides the combined advantages of a very wide spectral band coverage with all of the advantages of narrow band signal analysis. This processor can instantaneously channelize incoming signals in both frequency and one direction of arrival parameter. This wideband real-time signal processing is a major advantage over the prior art.
At the present time, there is no known technique in the art for designing a three or more dimensional acousto-optical processor. In view of the number of parameters involved in signal analysis, i.e. frequency, bearing, elevation, polarization, etc., this type of limitation is a major handicap to system design.